Implement parse_opt_bool better

During my clean-up of rebase errors, I took the opportunity to implement
parse_opt_bool so that it isn't identical to parse_bool wrapped in
`Some`.

parse_opt_bool considers no value to be true, a value of 'y', 'yes' or
'on' to be true and 'n', 'no' or 'off' to be false. All other values are
an error.

src/librustc/session/config.rs

@@ -348,7 +348,8 @@ macro_rules! options {
     #[allow(non_upper_case_globals, dead_code)]
     mod $mod_desc {
         pub const parse_bool: Option<&'static str> = None;
-        pub const parse_opt_bool: Option<&'static str> = None;
+        pub const parse_opt_bool: Option<&'static str> =
+            Some("one of: `y`, `yes`, `on`, `n`, `no`, or `off`");
         pub const parse_string: Option<&'static str> = Some("a string");
         pub const parse_opt_string: Option<&'static str> = Some("a string");
         pub const parse_list: Option<&'static str> = Some("a space-separated list of strings");
@@ -379,7 +380,19 @@ macro_rules! options {
 
         fn parse_opt_bool(slot: &mut Option<bool>, v: Option<&str>) -> bool {
             match v {
-                Some(..) => false,
+                Some(s) => {
+                    match s {
+                        "n" | "no" | "off" => {
+                            *slot = Some(false);
+                        }
+                        "y" | "yes" | "on" => {
+                            *slot = Some(true);
+                        }
+                        _ => { return false; }
+                    }
+
+                    true
+                },
                 None => { *slot = Some(true); true }
             }
         }